1. Field of the Invention
The present invention relates to methods of generating gas in and foaming well treating fluids during pumping of the treating fluids or after the treating fluids are placed in a subterranean zone, or both.
2. Description of the Prior Art
Foamed treating fluids have heretofore been utilized in a number of oil and gas well applications. Typically, a treating fluid is foamed by combining a mixture of foaming and foam stabilizing surfactants with the treating fluid on the surface followed by injecting gas, typically nitrogen, into the treating fluid containing the foaming and foam stabilizing surfactants as the treating fluid is pumped to the well head and into the well bore. This process allows the treating fluid to have gas concentrations of 1% to 80% by volume of the fluid depending on the downhole pressure and temperature and the amount of gas injected at surface. However, the equipment required for storing the nitrogen in liquid or gaseous form and injecting it into a well treating fluid is very elaborate and expensive. In addition, the equipment is frequently unavailable or cannot be easily transported to well sites due to their remote locations.
In situ gas forming agents have been utilized heretofore in well cement compositions to prevent annular gas migration. For example, surfactant coated finely ground aluminum has been included in cement compositions to generate hydrogen gas in the compositions as they are being pumped down a well bore and after they are placed in the annulus between the walls of the well bore and casing or other pipe string therein. The presence of the gas in the cement compositions prevents formation fluids from entering the cement compositions as the cement compositions develop gel strength. That is, the development of gel strength reduces the ability of a cement composition column to transmit hydrostatic pressure. If the hydrostatic pressure of the cement composition column falls below the formation pore pressure before the cement composition has gained sufficient strength to prevent the entry of formation fluids into the well bore, the fluids enter the well bore and form channels in the cement composition column which remain after the cement composition column sets. The presence of the gas which is generated in the cement composition from the finely ground aluminum increases the volume of the cement composition such that the volume increase generated by the gas equals or slightly exceeds the cement composition volume reduction during the development of gel strength due to fluid loss and/or the cement hydration reaction. The increase in volume and the compressibility produced in the cement composition by the gas allows the cement composition column to resist the entry of formation fluids into the well bore.
Other gas forming agents have also been added to well cement compositions to gasify the compositions. For example, U.S. Pat. No. 4,450,010 issued on May 22, 1984 to Burkhalter et al. discloses a well cementing method and gasified cements useful in carrying out the method. That is, U.S. Pat. No. 4,450,010 discloses a method of cementing in subterranean formations using a gasified cement composition which prevents formation fluids from entering the cement composition column formed in the annulus between the well bore and a pipe string therein. The cement composition includes a nitrogen gas generating material, an oxidizing agent, and a reaction rate control material whereby a quantity of gas is generated in the cement composition to offset the shrinkage in the cement composition column as it develops gel strength and to provide compressibility thereto whereby the entry of formation fluids into the well bore is reduced or prevented. While the methods and cement compositions of the prior art have achieved varying degrees of commercial success, there are needs for improved methods of generating gas in well cement compositions and other well fluids whereby the well fluids can be foamed in situ at various selected times during their use to produce a variety of desired downhole results. The control of the timing of in situ gas generation until after the well fluids are pumped also helps in preventing air-locking of the mechanical pumps used.
A situation where the presence of gas in a cement composition will provide a distinct advantage involves problems associated with high fluid pressure buildup behind cemented casing. Occasionally, drilling fluid and cement spacer fluids are left behind casing during the cementing of the casing in a well bore. When the well is put on production, the formation temperature heats up the trapped drilling and/or spacer fluids causing severe high pressure build ups due to the incompressibility of the fluids which can cause damage to the casing. The presence of a compressible gas behind the casing in drilling fluid and cement spacer fluids, either in the form of a gas pocket or foam, will help sustain the temperature increases without severe pressure buildups.